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Can a 4” APO ‘beat’ an 8” SCT? (yes and no)
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Can
a 4” APO ‘beat’ an 8” SCT? (yes and no)
by Gary S. Strumolo
This article is derived from a talk I gave my local astronomy club many years ago. This competition, and variations of it using different types and sizes of telescopes, seems to be a perennial question. Our goal in writing it was not to disparage any type of scope; all have their use and, like beauty, their value to an individual is in the eye of the beholder. We’ll not bring up issues like portability, weight, cool-down time, cost, or any of the many other factors that go into a purchase decision.
Rather, we’ll focus on the optics and their ability to render visually appealing images of both low and high contrast objects. We are not going to address what could be generated after recording, stacking, and processing thousands of video images. This analysis is not for astrophotographers; it’s for visual astronomers!
The tool we’ll use to conduct this analysis is called the modulation transfer function, or MTF for short. The MTF curve for an optical system, like a telescope, determines how much contrast of the object being observed is maintained after passing through the optical system. It can be illustrated here:
The MTF curve reflects the ratio of image contrast to object contrast.(This and all subsequent MTF curves were generated by the Windows program Aberrator).The type of curve above is typical for an unobstructed scope, like a refractor. If you compare two refractors of different apertures you get:
basically showing that the larger scope can reveal finer details on the object than the smaller one can. Aberrator can analyze more than an unobstructed lens, however:
In this example, we consider the case of an 8” aperture with a 33% central obstruction, as found in your typical 8” SCT. The downward bend in the curve reflects the fact that low contrast resolution is degraded because of the obstruction. To determine how much we consider the previous graph and ask how much smaller in diameter must the aperture be so that the new (unobstructed) curve will line up with this dropped down portion. That will give us the ‘effective’ size of an unobstructed scope that will match this one for low contrast features.
For example, consider this for an obstructed scope:
The black ideal curve is for an unobstructed aperture (refractor). The red actual behavior curve is for an obstructed aperture of the same size. The low contrast blue line, which matches the red curve drop, is for a smaller unobstructed scope. To determine its size we look to see where the blue curve hits the X-axis. Here it is around 0.7, which means the original scope performs like an unobstructed one 70% of it’s diameter.
The effects of a central obstruction on the ability to resolve low contrast features can be illustrated in the following diagram (20% for a typical Newtonian and 33% for a typical SCT):
So, e.g., the 20% obstructed scope (blue line) behaves like an unobstructed one 85% of its diameter (blue dashed line). Now obstruction isn’t the only factor in making the final decision about which is better. There are two others: collimation and seeing (turbulence). Let’s consider each, and how they are linked to the obstruction issue.
Collimation (for more, see T. Legault astrophoto.fr/collim)
Most people are familiar with the first phase of collimation, where you pick a star and deliberately rack it out of focus. You will see a set of rings (if you have an obstructed scope like a Newtonian, SCT or MCT there will be a dark central circle in the middle). The goal of ‘rough’ collimation is to make everything perfectly concentric. But that isn’t the end!
(and the bane of MCTs and Newtonians as well). The second stage is to use high powers on a focused star to see the Airy disks. These must also be concentric. As the figure above indicates, fig A is ideal while Fig D is still off. Now, how bad is it? Well, consider the pattern in Fig C above. It doesn’t look that bad, right? Well …
The figure above is an example for a Newtonian. We can see that that a C level of miscollimation produces an MTF curve that is similar to one from a spherical aberration of 1/3.5 wave, and one with a 43% obstruction. These are ‘equivalent’ to a 20% obstructed scope at 63% diameter (again, for resolving low contrast features).
So returning to our SCT:
we can see that while a perfectly collimated 8” SCT under excellent seeing conditions might behave like a 5.6” APO (left), poor collimation can reduce it to the level of of a 4” APO or worse! (hence the ‘yes and no’ in the title)
Seeing conditions (turbulence)
We will equate seeing conditions to turbulence in the atmosphere, ignoring things like poor transparency due to clouds, etc.. Now different aperture scopes are affected differently by turbulence:
(source:
https://skyandtelescope.org/astronomy-equipment/beating-the-seeing/)
This behavior might actually help the smaller scope over the larger one during visual astronomy because the eye can better follow detail as it moves a bit vs being steady but blurry. And unless you live in areas blessed with frequent steady skies …
But given the observation above, the effective turbulence for a larger scope is greater than that for a smaller scope. So we could be in the following situation:
So under fair-below average seeing conditions (which many people deal with most of the time) it’s possible for a reasonably well collimated 8” SCT to simply match a 4” APO.
So how do these analysis results compare to observations?
l An 8” beats a 4” on high-contrast objects (moon craters, Cassini division, shadow transits, edge of planet)
l A 4” can match (or possibly beat) an 8” on low-contrast objects (surface of Jupiter and Saturn, possibly Mars) except under very good-excellent seeing conditions.
To test this, we can use the Aberrator program to simulate the effects of turbulence on image quality:
We can see that under no turbulence the SCT beats the APO even for the low-contrast surface features of Jupiter, but when there is turbulence the image quality is equal.
I hope this helps explain the factors behind the quality of what you see in the EP. Of course, size matters, and if your goal is DSOs then the larger scope will always win out (after all, they are called ‘faint fuzzies’ for a reason). But, as we all know, the ‘best’ telescope is the one we use the most!
Thanks for reading and clear skies!
Gary S. Strumolo
- Mike McShan, B McCandless, ed100 and 21 others like this
91 Comments
In my experience with refractors of this size and larger, and my 8" Edge HD SCT, I would say that a 4" (100mm) refractor probably not. But getting on to 127mm to 152mm, in many cases yes because of the higher contrast and sharper images. Especially for lunar, solar, planetary, and double star visual astronomy, the refractors will win every time. They can hold their own with deep sky, though requiring longer exposures. But not as long as you might think: due to optical losses in SCTs, and the central obstruction, you don't get the equivalent of 200mm of clear aperture, more like about 175mm. Once I started using APOs, I never looked back, though I still have my 8" SCT. I use my 140mm f/7.5 CFF APO most of the time. My 115mm APM/LZOS will also give my 8" SCT a run for its money, falling down only at higher powers where contrast goes down due to the laws of physics. The APOs can be used with any focal length eyepiece, and are largely maintenance free with fairly quick cool down.
The downside of refractors is the cost per millimeter of aperture, compared to reflector designs, and weight per millimeter. My 140mm APO cost about 4x the 8" SCT. There is also the issue of the weight being at the top of the tube, necessitating a heavier mount, and the need to sit low or on the ground when observing overhead.
The downside of SCTs is the difficulty and frequency of collimation for optimum performance, and the tube currents inside that mean long cool down periods. I gave up on solar observing with SCTs because the tube currents for that type of target never did settle down. There is also a minimum focal length eyepiece with any reflector design, and lower contrast for visual observation. The long focal length of SCTs can also be a problem if you are looking at larger objects, or photographing them. Celestron makes a 0.7x focal reducer, which gets the 8" down to 1400mm, but it is for photography only, and will not cover a full-frame sensor. My refractors will do so natively and can have their focal length extended with Barlows (I recommend the TV PowerMates or Harry Siebert's 4-element Barlows).
Overall, it's likely to come down to a matter of $$, portability, intended use, and personal preference. An 8" SCT is very portable and moderately priced; an 8" refractor is neither. But a good APO in the 115mm to 130mm range is comparable to the SCT in terms of weight and size (though more expensive), and in most cases will meet or beat it in terms of contrast and sharpness, though not raw light gathering.
Hope this helps!
Financials and AP excluded:
If you're a visual only observer and like DSO's ---- take the 8" SCT.
If you're into the planets, close doubles, wide field opens like M45, etc. (again visual) - take a good 4" APO.
Short and sweet ....
True. That's why it was advanced to today. The article was submitted months ago
I'll be honest, if any of the 4" refractors I've owned and currently own gave eyepiece views as bad as those in the 4 images below, I'd either assume the seeing is very poor, or the scope wasn't up to the job. In my experience with refractors covering 43 years, when the seeing is stable they will deliver an eyepiece view very close to the one shown in the single Jupiter image alongside the four very poor images.
Below is the view as sketched through a good 4" refractor on a night of steady seeing from my home in the UK. I've only once seen an SCT deliver this kind of definition at the eyepiece, and that SCT was a 1980 orange C8 that had been standing in a cold observatory for three weeks. Perhaps it's my UK skies or perhaps it's the SCT design itself that's the problem, but if that's the case, why does every other major design of telescope that I've used beat the SCT hands down in terms of planetary definition, image sharpness and contrast, visually?
I've seen similar level of detail through my 4" as Mike's excellent sketch. But my 10" SCT shows significantly more detail, including swirls in the GRS, detail I've never even come close to seeing with my 4",
The images of Jupiter were generated by Aberrator to simulate the effects of turbulence on both scopes, keeping in mind that the turbulence for the SCT was twice that of the refractor, because of the cell size (as described in the article).
The sketch in my signature below was made at the EP of my C8, which came out of the house maybe 30 minutes before? Enough detail?
To your last question, perhaps the collimation on your SCT was off? Have you compared your scopes on a high contrast target like the moon at the terminator? And if the others are so much better why do premier planetary imagers like C. Go and T. Legault exclusively use SCTs for their work?
This question always degenerates into mish mash of confused technical discussion (present article is a nice exception to this) and subjective semantics. In my own experience doing side by sides with my TV102 and my C8 Edge, unless the seeing is really bad, or the SCT has not had enough time to cool, it wins. On a clear, still night, I have had the SCT up to ~400x on the moon, looking at craterlets inside of craters. The TV102 tops out at ~250 or so( which is still really good).
That said, I will have people routinely comment at star party open houses that my TV102 gives the most satisfying views on planets, compared to well collimated 10 and 14 inch Dobs with excellent optics sitting next to them. Having looked through those same dobs myself many times, I can say with certainty that my TV102, as much as I love it, does NOT out perform a 10 inch dob, or even my c8 in most cases. So what are these folks talking about? I believe it has more to do with the ergonomics of my setup, and the fact that I insist that they take their time looking through the eyepiece. They see more, because they LOOK more and are comfortable when they do it. Observing is an experience that involves more than just the optics.
JMD
Gary:
I was expecting the typical in the field comparison where objects are chosen that play into the hands of one scope or the other. However, I was very pleasantly surprised by your careful and unbiased analysis..
Jon
So, as long as at least one or two things go completely wrong (seeing, collimation) an 8 inch might (on low-contrast detail) match a 4 inch refractor. But for most deepsky, the 8 inch produces a more detailed image virtually all of time. Remember when observing planets, we'd wait for fleeting seconds when the atmosphere steadied out so we could see our (larger) scopes meet their potential? That is the idea, excellence where possible, not just passable images most of the time.
Some day someone will compare a 4” apo to a 4” reflector/sct (or 6” vs 6” or 8” vs 8”) for a truly unbiased comparison…but of course we already know the outcome there.
As usual, what this comes down to is......cost.
Something that deservedly costs 2 to 4 times (and more) should be better, in all things. So looked at from that perspective, it is a credit to the designers and manufacturers that SCTs are able to compete with the high-dollar refractors at all.
It depends on your criteria for being 'unbiased'. You seem to have selected aperture size as the constant (while keeping the APO for the refractor I see). Of course, comparing an unobstructed APO scope to obstructed ones of the same aperture is an unfair comparison. But how do you think a 6" or 8" achromat would fare against a similar SCT or Newtonian. What if we chose cost as the constant? Could you even find (forget about afford) an 8" APO? I chose the two contenders because they are often put up against each other, with each side defending their position with an almost religious fervor. I personally have at least one of each: refractor, Newtonian, SCT and MCT. I love them all, and each has its particular advantage depending on the target.
Refractors are all over the place. Compare an ST-80 or a 102mm F/5 achromat to a 4 inch Mak on the planets and you will get one outcome. Compare a 4 inch F/7 FCD-100/Lanthanum doublet and the outcome will be quite different.
Comparison based on equal apertures never really make much sense. When one is choosing a scope, one does not decide between an 8 inch refractor and an 8 inch SCT... Dollars, hassle factor, pound for pound, these make sense.
Jon
Curious if you have your full presentation available for viewing?
great information!
Jerry
I agree with Jon Isaacs above. I was pleasantly surprised to see this article pop up as I just recently decided that if I can land a good NP101 like the one I had a few years ago, I'm going refractor only. I know (from plenty of experience) that ap fever will still nag at me, but so many factors have to be taken into account. Your age and physical abilities also come into play (the reason I recently and reluctantly sold my 9.25" EdgeHD which because of shoulder pain I had to accessorize after lifting onto my mount then de-accessorize before lifting it off the mount each session), your financial situation, whether you plan to just do visual, or AP, or both, your atmospheric conditions and light pollution factors, how much trouble you want to go to keeping optics clean and collimated... Some of this was covered here very nicely.
The two best scopes (for me, and my purposes) that I've owned over the years were a TV NP127is and the 9.25" EdgeHD. If I could, I'd still have both, but so many factors come into play, including, and maybe you'll laugh at this, but I'm serious, the thought that if this aging guy DROPPED one of those scopes when setting up or tearing down, that would be VERY painful in more than one way.
In any event, thanks for this article, which for me was especially timely!
Glad to help!
On a side note, your concern about dropping a scope is why I keep my Astroscan. When I go to a dark site I always worry about someone bumping into my setup and the scope heading to the ground. But since the Astroscan is built like a tank I pity the ground more! And its 4" of aperture coupled with a 3 degree FOV @ 16x makes scanning the skies a lot of fun.
The Astroscan is in a category all by itself, that's for sure. Yes, built like a tank, for sure. I picked up a cracked one once and figured I could break it down and sell the parts, and that was a job and a half taking that thing apart!
My experience in the early 80's using a 4" Unitron- obviously not an APO vs. a 10" well regarded Cave deluxe Newtonian revealed that the Unitron was surprisingly capable of matching the bigger scope. Certainly, for DSO the Uni was overmatched, but for lunar and planetary observations, it was difficult to say which provided better views.
I'd like to know more about the atmospheric RMS (root-mean-square) fluctuations expressed above. How does .05 or .10 RMS correlate to the Pickering scale or some other measurement of turbulence?
If you input "Can a 4” APO ‘beat’ an 8” SCT?" into the search function under "forums", there are innumerable replies along the same lines.
And your point is?
But do any of them come at it from the standpoint of the MFT? I can't search through innumerable threads to find out - too many! My goal was not to enter a holy war between the two camps but to try to present an unbiased analysis (as others have noted) to what is an emotional issue for some (God only knows why; they're just telescopes, folks!). I hope you enjoyed it!
These kinds of articles have been coming out for years. I have a little experience on the subject as I have been playing with scopes for over 50 years and build them and modify them and compare them and have read most every book on design over the last 100 years and currently have (at least) 30 scopes of all types from 60mm refractors to a 16" equatorial Newt, SCT's Maks, Achromats and APO's, etc., and I like to compare them.
First, one must assume the SCT has decent optics (I have 5 with excellent optics but there are lemons in mass produced scopes) and is perfectly collimated. It is easy and if the screws are torqued correctly they rarely need to be tweaked. Next they MUST be thermally equilibrated- the biggest problem. All SCT's need a thermal blanket (reflectix is perfect) or the tube is radiating to the sky creating tube currents falling, and the mirror is thermally connected to the baffle tube which usually creates a plume there with warm air trapped behind the corrector. My C 8" , 11" and !4" have vents either in the rear cell or tube near the primary AND just behind the corrector and as long as the initial delta T is not too wild they all (including the C14) come to thermal equilibrium in minutes and stay equilibrated as the temperature drops.
As for refractors I have APO 80's, 120's and 150s and a bunch of achromats.
Now what objects are we going to look at? I mean how many things visually are there to look at (imaging is another subject entirely). Well, 7 planets (and a few moons) the Moon, maybe 100 Messier objects (need a really dark site for the rest with these small scopes), a few NGC objects Messier missed, maybe 100 doubles (the Sun is a separate subject)(what did I miss?). The 8 will beat the 4" on EVERY object in good seeing, period. Now what object really is the test? In my mind there is only two objects with a lot of low contrast detail that stands out here- Jupiter and Mars (Saturn doesn't have that much low contrast detail)(a 5"APO will about equal the C8 on planets). That's it. When the seeing worsens small refractors are affected but not as much, so a 4" APO will equal or even beat beat EVERY big scope (even 10's and up), but it will still suck LOL. On everything else in good seeing bigger aperture will win. (and a 8" f/5 or/6 Newt will do even better and is less expensive-care and feeding is just different). But the C8 is so compact, the pier/ tripod doesn't have to be very high and a EQ5 series mount is adequate, the wind loading is low, the weight is similar, can get a good used one for $300.
OK enough LOL. Still a good article.
Thanks! And much of what you say is in the article: under no turbulence an 8" SCT behaves like a 5.6" APO (for low contrast objects; for high it is like an 8"); for higher turbulence the smaller scope is better for the reasons outlined above, but either view may not be that great, etc.
But I was attempting to quantify more the impact of turbulence and level of collimation on how they might counter the benefit of size advantage in a way I haven't seen in those many articles.
In my book, the only constant I like to keep is the quality of my observing time vs amount of money spent. And for that, here's another perspective: instead of buying a 4 inch APO, why not get a dirt-cheap $200 6 inch tabletop dob, and use the rest $800 or so for trips to dark-sky sites (or to sites with excellent seeing) and enjoy the tenfold better views.
The tabletop dob will do at least 80% of everything the refractor can do. Some examples may even be better than the refractor.
if you've got a 8 inch or 10 inch Dob, then its no comparison.
There's very little bias towards Newts/Cats. In most reviews I've read, the bias is actually against them.